Making a statement may be the paradigmatic use of language, but there are all sorts of other things we can do with words. We can make requests, ask questions, give orders, make promises, give thanks, offer apologies, and so on. Moreover, almost any speech act is really the performance of several acts at once, distinguished by different aspects of the speaker's intention: there is the act of saying something, what one does in saying it, such as requesting or promising, and how one is trying to affect one's audience.
The theory of speech acts is partly taxonomic and partly explanatory. It must systematically classify types of speech acts and the ways in which they can succeed or fail. It must reckon with the fact that the relationship between the words being used and the force of their utterance is often oblique. For example, the sentence 'This is a pig sty' might be used nonliterally to state that a certain room is messy and filthy and, further, to demand indirectly that it be straightened out and cleaned up. Even when this sentence is used literally and directly, say to describe a certain area of a barnyard, the content of its utterance is not fully determined by its linguistic meaning--in particular, the meaning of the word 'this' does not determine which area is being referred to. A major task for the theory of speech acts is to account for how speakers can succeed in what they do despite the various ways in which linguistic meaning underdetermines use.
In general, speech acts are acts of communication. To communicate is to express a certain attitude, and the type of speech act being performed corresponds to the type of attitude being expressed. For example, a statement expresses a belief, a request expresses a desire, and an apology expresses a regret. As an act of communication, a speech act succeeds if the audience identifies, in accordance with the speaker's intention, the attitude being expressed.
Some speech acts, however, are not primarily acts of communication and have the function not of communicating but of affecting institutional states of affairs. They can do so in either of two ways. Some officially judge something to be the case, and others actually make something the case. Those of the first kind include judges' rulings, referees' calls and assessors' appraisals, and the latter include include sentencing, bequeathing and appointing. Acts of both kinds can be performed only in certain ways under certain circumstances by those in certain institutional or social positions.
The philosopher J.L. Austin (1911-1960) claims that many utterances (things people say) are equivalent to actions. When someone says: “I name this ship” or “I now pronounce you man and wife”, the utterance creates a new social or psychological reality. We can add many more examples:
Speech act theory broadly explains these utterances as having three parts or aspects:locutionary, illocutionary and perlocutionary acts.
Some linguists have attempted to classify illocutionary acts into a number of categories or types. David Crystal, quoting J.R. Searle, gives five such categories: representatives, directives, commissives, expressives and declarations. (Perhaps he would have preferreddeclaratives, but this term was already taken as a description of a kind of sentence that expresses a statement.)